Vehicles with an internal combustion engine may be fitted with fuel vapor recovery systems wherein vaporized hydrocarbons (HCs) released from a fuel tank are captured and stored in a fuel vapor canister containing a quantity of fuel-absorbing material such as activated charcoal Eventually, the fuel vapor canister may become filled with an amount of fuel vapor. The fuel canister may be cleared of fuel vapor by way of a purging operation. A fuel vapor purging operation may include opening a purge valve to introduce the fuel vapor into the cylinder(s) of the internal combustion engine for combustion so that fuel economy may be maintained and fuel vapor emissions may be reduced.
Activated charcoal has been found to be a suitable fuel vapor absorbing material to be used in such a canister device because of its extremely porous structure and very large surface area to weight ratio. However, this porous structure can be blocked and lose its efficiency when coated with liquid fuel. This may occur if, for example, during refueling a pump operator adds fuel after an initial automatic shut-off. For instance, in an attempt to maximize the amount of fuel pumped into the tank, a pump operator may dispense additional fuel in what is commonly referred to as “trickle-filling”. If liquid has entered the evap recovery lines and a purge cycle is commanded at the next engine start, the liquid can get sucked into the canister and corrupt the activated carbon. This may damage the canister and lead to increased HC emissions. Additionally, if liquid fuel in the canister or purge line is purged to the intake, a loss of engine power may result from an extremely low air-fuel ratio (A/F). Accordingly, it is imperative to easily diagnose and mitigate the presence of liquid fuel in the evap recovery lines.
Toward this end, US Patent Application US 2007/0131204 A1 teaches a method of detecting whether liquefied fuel exists in a canister purge line based on a fuel level in a fuel tank higher than a pre-set level. If the fuel level is greater than a pre-set level, an air ratio is measured by an oxygen sensor in the exhaust manifold. The purge control valve is then opened for a pre-set time period, the air ratio is measured again, and a difference is calculated between the two. If the difference is less than or equal to a first value, and the air-fuel ratio after opening the purge control valve is less than or equal to a second value, then it is deemed that liquefied fuel exists in the canister purge line. If liquefied fuel is deemed to exist in the canister purge line, the purge control valve may be closed for a pre-set time period. Thus, loss of engine power due to a low A/F caused by liquefied fuel in the canister purge line is prevented by detecting liquefied fuel in the canister purge line in advance. However, the inventors herein have recognized potential issues with such a method. For example, the method is such that detection of fuel in the canister purge line does not alleviate or prevent the possibility of vapor canister degradation due to liquid in the vapor canister. Further, the method does not provide mitigating actions to purge liquid fuel from the evap recovery lines. An attractive alternative therefore, is a method that includes both detection and mitigation of fuel carryover in the evap line(s), such that liquid fuel does not come into contact with the activated charcoal housed within the vapor canister.
In one example, the issues described above may be addressed by a method for a vehicle. During a first condition, a vacuum pump is activated to pressurize the fuel system responsive to a first fuel tank pressure decay rate being less than a threshold and, responsive to a second fuel tank pressure decay rate being greater than a threshold, the vacuum pump is maintained on until a fuel tank pressure decreases to atmospheric pressure. In this way, fuel carryover in an evap recovery line may be accurately diagnosed and mitigated, such that canister degradation due to liquid fuel contacting the adsorbent is avoided.
As one example, responsive to the second fuel tank pressure decay rate being less than the threshold, the vacuum pump may be deactivated and a restriction may be indicated in the evaporative emissions system coupled to the fuel system. In this way, pressure build-up due to the vacuum pump is avoided, and mitigating action may be performed to diagnose and remedy the indicated restriction.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.